Move comment to the right place
Merge pull request #1 from dfellis/master

Add true immutability to the library.
Add extra check before defineProperty

Also had to make sure the property is enumerable (it's not by default).
Deletion still works even though it shouldn't, though.
Set up travis
Making sure 'set' produces a truly immutable object, and fixing a couple of places where mutation was being used in the library.